United States military Veterans from recent conflicts are coping with symptoms related to mild traumatic brain injury (mTBI), persistent post concussive (PPC) symptoms, and posttraumatic stress disorder (PTSD). Many Veterans are resistant to conventional health and mental health interventions (e.g., medication, psychotherapy), and often symptoms are not significantly improved by traditional treatments. Moreover, there are limited treatments for symptoms associated with both conditions, which frequently co-occur. Alternative treatment methods are needed. One potential common underlying feature of both mTBI and PTSD is exaggerated inflammation, both peripherally and in the central nervous system, which is thought to play an important role in the vulnerability to, aggravation of, and perpetuation of adverse consequences of these often co-occurring conditions. Therefore, a novel intervention strategy would be the use of immunoregulatory/anti- inflammatory probiotics to reduce inflammation. In this study, we will investigate the effects of an immunoregulatory probiotic on both biological signatures of systemic inflammatory processes and proximal signatures of probiotic administration. Lactobacillus reuteri (L. reuteri), a commensal organism that colonizes the human gut mucosa, suppresses mucosal inflammation via inhibition of the production of proinflammatory cytokines, and is the probiotic of interest. Specific aims of the study are to determine the: 1) effects of L. reuteri on biological signatures of gut microbiota, gut permeability, systemic inflammation processes, and stress responses; 2) feasibility of L. reuteri supplementation; 3) acceptability of L. reuteri supplementation; and 4) tolerability and safety of L. reuteri supplementation. Project aims will be assessed using a longitudinal, double blind, randomized placebo-controlled design. Participants will be Veterans with PPC symptoms, PTSD, and evidence of elevated systemic inflammation (based on high baseline plasma C-reactive protein [CRP] concentrations). After initial evaluation procedures, 20 participants will be randomized to probiotic supplementation and 20 will be randomized to placebo supplementation. The proposed line of research addresses the Office of Rehabilitation Research and Development (RR&D) Service's goal of identifying means of intervening to increase function among those with mTBI and co-occurring psychiatric conditions. Long-term, this study may lead to a paradigm shift in the manner by which we target clinical symptoms associated with PPC and PTSD symptoms, by beginning the process of supporting a multitargeted, neuroprotective approach.